The glucose oxidase polarographic peroxide electrode (Clark glucose sensor), widely used for laboratory blood glucose measurements for over a decade, will be modified so as to be implantable in tissues or on organs. This electroenzymatic sensor is highly specific for glucose, and is not subject to interference by ascorbic or uric acids, by bilirubin, or by commonly employed drugs. The Clark glucose sensor has been used to measure subcutaneous and cortical glucose for periods of hours or days. In the laboratory stability of the enzyme, membrane and the electrode is high. Our aim is to develop an implantable electrode with a clinically acceptable functioning lifetime. The Clark glucose sensor,, modified for in vivo work, will be implanted subcutaneously, intramuscularly, and on the liver surface of cats. Current outputs will be recorded daily for several months or until the electrode fails or is sluggish in response. The best method for in vivo calibration of implanted glucose sensors canabe perfected by this means. Periodic glucose tolerance curves will be run where both implanted glucose sensor current and blood glucose levels will be measured, recorded, and compared. From this information the best location for an implanted sensor will be discerned, the best configuration worked out, and a reliable standardizable, implantable glucose sensor suitable for use in an artificial pancreas will be perfected.